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Can we grep how x$ structures are mapped to v$ views ?>

NO - because, in general, X$ structures are NOT mapped directly to V$ views; they are generally mapped to the GV$ views and those GV$ views are what is mapped to the V$ views.

And why, on Earth, would you want to use grep rather than just query the info from Oracle? If you want to know the definitions of the fixed views just use the views that Oracle provides for that very purpose: V_$FIXED_TABLE and V_$FIXED_VIEW_DEFINITION.

The X$tables are FIXED-TABLES. Oracle created V$VIEWS on the X$ TABLES to make it easier to see Oracle internal details.

SQL> select synonym_name, table_name from dba_synonyms

2 where synonym_name in ('V$VERSION', 'GV$VERSION' , 'V$SESSION');

SYNONYM_NAME TABLE_NAME

-------

V$SESSION V_$SESSION

V$VERSION V_$VERSION

GV$VERSION GV_$VERSION

QUESTION : Here any V$views doesn't belong X$tables. - Why ??

Thanks.

I am not sure that I understood you. The views based on X$ tables are actually named as V_$. The V$ ones are merely their synonyms. What do you mean by "v$views doesn't belong x$tables" ?Which V$ views you have found which are not made up from a X$ base table(s) ?